Best way to require Haml on Rails3 engines - ruby-on-rails

Question

I'm developing a Rails3 engine application, and I want to use Haml for the views.
First, what I have done was to add this to the engine Gemfile:
gem "haml"
While I was testing my engine, it was working OK (I have used https://github.com/josevalim/enginex to generate the gem and test it with the dummy application).
My problems started when I tried to use the engine on a real Rails application. The application does not have gem "haml" on it's own Gemfile, and so it was not initializing Haml, so I was receiving template not found errors as it was not looking for the .haml views. I was thinking that by requiring Haml on the Engine it would be enought for it to be also required by the Rails application.
What I have done for now was to add a config/initializers/haml.rb on the engine with this code:
require 'haml'
Haml.init_rails(binding)
It's working now, but I'm wondering if this is really a good way to do it.
Why Rails is not calling Haml "init.rb" file and so initializing Haml correctly by just adding gem "haml" to the engine Gemfile?

Answer 1

Two things are necessary. First, in the .gemspec:
s.add_dependency 'haml', ['>= 3.0.0']
And in your lib/gem_name.rb:
require 'haml'
And then run bundle both inside the gem and app directories.

Answer 2

I think you will have to put haml in the engine gemspec as a dependency in order for bundler to install haml in the target application (and show up in its Gemfile.lock). Something like this:
Gem::Specification.new do |s|
s.add_dependency(%q<haml>, [">= 0"])
end
I just tested this out on one of my engines. Without the dependency in the .gemspec it did not install haml in the target app (did not appear in Gemfile.lock). After I added haml to the gemspec as a dependency, it does show up:
PATH
remote: /rails_plugins/mine/my_engine
specs:
my_engine (0.0.0)
formtastic
haml
inherited_resources
settingslogic
sqlite3-ruby
GEM
remote: http://rubygems.org/
specs:
#................
haml (3.0.25)
#................
If you are using jeweler, it will add the dependencies to the gemspec automatically based on what is in your Gemfile.. it even adds a developement_dependency if you have the group defined in your Gemfile. I have only looked at enginex briefly, so I don't know if it has a similar rake task to build the gemspec.
This might help clarify some things:
http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/